feat(schema) add hidden field feature to hide selected schema fields

[artomal]

Summary

Add feature that enables engineers to hide chosen fields in schema from the clients that use API. There are a couple of reasons to have this feature, one of the examples would be to hide "operational" system fields from the end client like calculating digest values of certain fields that is only used internally.

Full changelog

• Add new metaschema attribute hidden boolean type
• Add related tests

[Tieske]

Is this needed on the database level? If the values are calculated, can't we just calculate them once, when loading from the db?

Like a de-serialization callback in ml-cache for example. See the option l1_serializer here: https://github.com/thibaultcha/lua-resty-mlcache/blob/master/README.md#new